<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        html {
            scroll-behavior: smooth;
        }

        body {
            height: 2000px;
        }

        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            /* display: none; */
        }

        .container {
            width: 900px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
    <script src="jquery.min.js"></script>
</head>

<body>
    <div class="back">返回顶部</div>
    <div class="container">
    </div>
    <script>
        $(function () {
            // 返回顶部
            $('.back').click(function () {
                $(document).scrollTop(0)
            })

            // 被卷去的头部scrollTop() / 被卷去的左侧 scrollLeft()
            //页面滚动事件
            let boxTop = $('.container').offset().top
            $(window).scroll(function(){
                if($(document).scrollTop() >= boxTop){
                    $('.back').fadeIn()
                }
                else{
                    $('.back').fadeOut()
                }
            })

        })
    </script>
</body>

</html>